The enzyme acetyl choline esterase converts acetylcholine into the inactive metabolites choline and acetate. This
enzyme is abundant in the synaptic cleft, and its role in rapidly clearing free acetylcholine from the synapse is
essential for proper muscle function. Certain neurotoxins work by inhibiting acetylcholinesterase, thus leading to
excess acetylcholine at the neuromuscular junction, causing paralysis of the muscles needed for breathing and
stopping the beating of the heart.
In the brain, acetylcholine functions as a neurotransmitter and as a neuromodulator. The brain contains a number
of cholinergic areas, each with distinct functions. They play an important role in arousal, attention, memory and
motivation.
Partly because of its muscle-activating function, but also because of its functions in the autonomic nervous
system and brain, a large number of important drugs exert their effects by altering cholinergic transmission.
Numerous venoms and toxins produced by plants, animals, and bacteria, as well as chemical nerve agents such
as Sarin, cause harm by inactivating or hyperactivating muscles via their influences on the neuromuscular
junction. Drugs that act on muscarinic acetylcholine receptors, such as atropine, can be poisonous in large
quantities, but in smaller doses they are commonly used to treat certain heart conditions and eye problems.
The addictive qualities of nicotine derive from its effects on nicotinic acetylcholine receptors in the brain.